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Acid Deterioration

The paper described in the Cellulose section and depicted in illustration 8 is a completely pristine sheet made entirely from long chain, alpha cellulose fibers with no additives or impurities of any kind. Alpha cellulose is the pure, long chain cellulose depicted in illustration 5. Unfortunately, most paper available today contains a variety of additives, impurities and other less stable plant products which cause acid deterioration of paper. Other culprits which also have a deleterious effect on paper are environmental and atmospheric acids and pollutants. As you may have surmised when reading about the construction of the paper fiber, the destruction follows essentially the same route but in the reverse direction. Acids attack the bonds which hold together the glucose rings, the cellulose chains, the microfibrils, the bundles and the fibers.

What is an acid? A simplified, but acceptably accurate description is that an acid is any substance which can donate a proton. Earlier it was mentioned that the hydrogen atom is the only element which has only one proton in the nucleus and one electron in orbit. When hydrogen loses that negatively charged electron, it becomes positively charged (an ion), consisting of only one proton. This proton is strongly attracted to negatively charged electrons which overlap and share outer energy levels or orbits with other atoms to form the chemical (in this case, covalent) bonds which hold the long chain, cellulose molecule together.

The oxygen atom (0), shown connecting the two glucose units (rings) in illustration 9 has formed a covalent bond by sharing the six electrons in its outer (L) orbit with one electron from each carbon to form a stable outer orbit of eight electrons. The two hydrogen atoms each share their single electron with the three electrons each carbon atom has left. Combined, this provides another stable outer orbit of eight electrons. Now an acid (a hydrogen ion - proton [H+]) is introduced (see illustration 10).

The positively charged hydrogen ion + (acid) is strongly attracted to a negatively charged electron. The hydrogen ion combines with one of the electrons being shared between the outer energy levels or orbits of the carbon and oxygen atoms. The hydrogen atom now shares this electron with the oxygen atom, breaking the bond between the two glucose units or rings of the cellulose chain (see illustration 11). Now, instead of a single, long chain there are two shorter, weaker chains. The right side of the ring is stable because by sharing the electron from the hydrogen atom, the outer orbit of the oxygen atom still contains eight electrons.

The left side of the chain, however, is not stable. The hydrogen ion combined with one of the carbon atoms electrons leaving the carbon atom with only five electrons. This loss of one negative electron means the carbon atom now has a positive charge, so it is now a carbonium ion. The positively charged carbonium ion now seeks to achieve the same stability possessed by the right side of the ring shown in illustration 11. The presence of a water molecule will provide the opportunity for the carbonium ion, and the left side of the ring, to become stable (see illustration 12).

The positively charged carbonium ion accepts a negatively charged electron from the water molecule. This electron is shared between the outer orbits of the carbon atom and the oxygen atom. The left side of the ring is now also stable, having returned to the same number of electrons (as shown in illustration 10). However, the electron now being shared between the outer energy levels or “orbits” of the oxygen and carbon atom was taken from the hydrogen atom. This leaves a free hydrogen nucleus (which is a proton or acid) (see illustration 13).

The hydrogen ion (acid) that was released, will break another covalent bond connecting the rings of a cellulose chain, which will release yet another hydrogen ion. As the chain is broken into successively shorter lengths, it becomes progressively weaker. When one half to one percent of the bonds are broken the paper will be virtually useless. When the cellulose chain is broken, it also weakens and often breaks the hydrogen bonds which bind the ribbons, or chains, into sheets. The layers held by Van der Waals forces suffer the same fate. The hydrogen bonds are relatively weak, having a bond strength of 3 to 6, compared to the bond strength of 86 for the carbon-oxygen bond shown in illustrations 9, 10, and 11. The hydrogen bonds strength comes from the close proximity of the hydrogen atom to the oxygen atom.

The geometry of the covalent bonds connecting the rings in the cellulose chain is such that the hydrogen atoms are forced into a certain plane close to the oxygen atoms. A long chain results in a stronger, more rigid structure with higher strength hydrogen bonds. As the chain is broken into shorter and shorter lengths, this rigidity is lost. The hydrogen and oxygen atoms are no longer forced into planes of close proximity and the bonds can progressively weaken and break. Like the hydrogen bonds, Van der Waals forces are weak (with a bond strength of 2 to 10) relative to the covalent bond holding the rings in the cellulose chain together. Also, like the hydrogen bond, Van der Waals forces are weakened and broken when the covalent bonds connecting the rings break chemically (by acid). The strength of Van der Waals forces are also dependent on the geometry of the short carbon-hydrogen bonds, which minimize the distance and, therefore, maximize the strength between the layers. As the chain is broken and rigidity is lost, the carbon-hydrogen bonds are no longer so strongly forced into the geometric plane which keeps the layers at a minimum distance from each other. A loss of strength is then suffered in the bonding between the layers.

This combination of interrelated forces and chemical reactions is the primary cause of the massive amount of deteriorating paper artifacts found in libraries and archives throughout the world today.

Hopefully, you now can understand not only the devastating effect acid has on paper, but the mechanism via which this deterioration occurs.